External circulating system for pulp digesters



Oct. 15 1940. K, E

EXTERNAL CIRCULATING SYSTEM FOR PULP DIGESTERS Filed Dec. 51

INVENTOR ATTORNEY .BY I

Patented Oct. 15, 1940 UNITED STATES PATENT OFFICE EXTERNAL CIRCULATINGSYSTEM FOR PULP DIGESTERS poration of New York Application December 31,1937, Serial No. 182,693 6 Claims. (C 92-7) My invention relates toimprovements in external circulating systems for pulp digesters such asare employed in the paper making industry. Such systems are an essentialpart of an indirect heating system wherein the heat required for thecooking of the sulphate, or soda pulp and liquor is obtained bywithdrawing liquor by suction from the pulp being treated, passing thewithdrawn liquor in indirect contact with a heatmg medium such as steamin suitable heat exchange apparatus, and then returning the heatedliquor to the body of pulp, through which it circulates and gives upheat. This cycle is carried on during the cooking process, and it hasbeen found that the real effectiveness of indirect heating systems ofthis kind depends upon a uniform liquor circulation. However, externalcirculation may also be used with pulp digesters employing directheating.

My invention particularly pertains to an improved form of suctionstrainer through which the liquor that is withdrawn from the pulp in thedigester shell passes, and to other means hereinafter to be described,whereby the circulating liquor iskmore uniformly brought in contact withthe mass of pulp in the digester, which is thereby heated more uniformlythan has heretofore been the case, to the desired temperature, thisbeing true whether or not the pulp and 30 liquor are initially exposedto indirect, or to direct contact with a heating medium. I

The usual form of suction strainer consists essentially of a hollow ringso apertured as to admit liquor only thereinto, and which is suitablysupported in a horizontal position on the inner wall of the digestershell containing the pulp and liquor mixture. A strainer of this generaltype is disclosed in my copending U- S.

application Serial No. 138,333, filed April 22, 1937.

One or more suction lines connect the annular space inside the strainerwith a circulating pump for the passage of the liquor that is drawnthrough the perforations in the wall of the strainer. It has previouslybeen customary to 45 arrange these perforations on a uniform pitch overthe entire length of the ring. For simplifying piping it is frequentlydesirable to have a single suction connection to the circulating pump,and since the cross-sectional area of the strainer 50 must necessarilybe rather limited in order not to occupy too much space within theshell, the velocity of the liquor as it passes through the wall of thestrainer is relatively high close to the suction connection, while it isprogressively lower- 55 as the distance from the suctionconnectionincreases. The velocity of liquor within the ring progressivelyincreases as the suction connection is approached. The efiect of thisincrease in velocity upon approaching the point of suction, with theaddition of the frictional loss within 5 the ring, is to bring about arelatively steep hydraulic gradient between the inside and outside ofthe strainer at a point close to the suction connection with aprogressively decreasing gradient as the distance from the suction con-10 nection increases. This causes a non-uniform flow through the severalparts of the strainer with the result that the portions of the digesteraway from the suction connection do not receive a proper amount ofrecirculated liquor and the 15 pulp in these portions is under cooked".

In accordance with my invention the suction strainer is designed withperforations or holes spaced on a relatively large pitch adjacent thesuction connection and on a progressively de- 20 creasing pitch as thedistance from the suction connection increases, the spacing of theperforations being such that with the existing pressure difierence atany point in the strainer the flow of liquor through the wall of thestrainer per 'lineal foot of its circumference will be substantiallyuniform. Also special means may be provided whereby the heated liquorreturned to the shell will be distributed more uniformly over the entirecross-sectional areas of the digester.

For a better understanding however of all features of my invention,reference may be had to the following description considered inconnection with the accompanying drawing forming a part thereof, and inwhich:

Fig. l is a more or less diagrammatic view in elevation of a pulpdigester and indirect heating system therefor embodying the features ofmy invention;

, Fig. 2 is a horizontal sectional view taken on line 22 of Fig. 1illustrating the suction strainer 40 in plan;

Fig. 3 is a vertical sectional view showing the strainer outlet, takenon line 3-3 of Fig 2; and

Fig. 4 is a horizontal sectional view taken on line 4-4 of Fig. 1showing a return flow fitting of volute or spiral form, which is anotherfeature of my invention.

Like characters of reference referto' the same parts throughout theseveral 'views.

Referring to the drawings, reference character Ill designates acylindrical digester shell adapted to contain the pulp and (liquor.During operation, liquor is withdrawn through draw-ofl suction pipe H,which is connected to an outlet vided with suitable flanges to whichillustrated therein, is annular nozzle l2 of a suction strainerindicated generally at I3, which prevents the withdrawal of pulp orchips from the shell. From the draw-01f pipe II the liquor flows throughline IE to the suction of pump l by which it is then forced through lineis to the lower end of a two-pass heater I! of the shell and tube type.Steam or other heating medium is introduced into the heater at its upperend through line it, and is discharged from the lower end of the heaterthrough line is. After passing through the heater the liquor flowsthrough line 2d, and is returned to the upper and lower portions of thedigester through lines 28 and 23 respectively which are controlled byvalves 36 and 3? respectively. A perforated spray ring 22 which isconnected to line it serves to evenly distribute the heated liquorflowing therethrough over the pulp or chips in the upper portion of theshell, while a return fitting 2A, in the form of a volute or spiral, asclearly illustrated in Fig. 4 of the drawing, is so designed that thereturning heated liquor from line 23 will be forced circumferentiallyaround the inner wall of shell inlet opening 25, thus giving the liquora whirl, the centrifugal force of which will spread the stream of liquormore eifectively across the interior of the shell than would be possibleif the liquor were introduced in a stream with all elements of flowparallel.

Referring to Fig. 2, the suction strainer in form, conforms to thecontour of the inner wall of the digester shell and consists of aplurality of arcuate sections Eli and Ziia, each of which has atransverse cross-sectional configuration in the form of a right angledtriangle clearly shown in Fig. 3 of the drawing, providing a channel 35.Except for the section 260, which is provided with the chan nel outletopening formed by a sleeve il that passes through the wad of the shelliii, and is received by the suction nozzle id in sliding rela tionshiptherewith, the outer wall 28 of each of the sections 26 is imperforate,whereas the inner vertically inclined wall 29 and the lower horizontalwall 39 are perforated by a plurality of holes 39. The several walls arewelded together as indicated and the sections are secured in position onthe inner wall of the digester shell ill by supporting members, each ofwhich includes a straplike clamp 32 and a backing plate indicated at 33.The clamps 32 are shaped to conform to the general contour of the walls29 and 3d and the back ing plates 33 are welded to the shell and areprothe clamps are secured as by bolts 3 51. Adequate spacing is providedbetween the adjacent ends of the strainer sections to permit freeexpansion and contraction of the strainer sections: relative to eachother and to the digester shell. A plurality of spaced stiffener members38 are secured as by welding, to the outer face of wall 29.

As will be seen from an inspection of the drawing, the perforations orholes 3| in the strainer walls are spaced on a relatively large pitchnearest the point in the strainer of greatest suction, i. e., thesuction outlet 21, and are spaced on a progressively decreasing pitch asthe distance from the suction connection increases. The spacing of theholes is such that for the existing pressure difi'erence at any point inthe strainer, the

flow of liquor from the digester through the holes in the walls of thestrainer will be substantially uniform throughout its circumference, andhence there will be a uniform circulation, or substantially uniformcirculation, of liquor through the body of pulp in the digester, and allportions of the pulp will be evenly heated or cooked.

Additionally, by means of the spiral return fitting 24, that portion ofthe liquor returned to the lower portion of digester, will bedistributed uniformly over the contents contained therein. However, itis to be understood that the invention does not necessarily consist ofthe combination of both my improved form of strainer and the volute orspiral return fitting, as either of these features may be employedseparately if desired.

Although I have described and illustrated a preferred form of myinvention in connection with an indirect heating system, it will beunderstood that changes or modifications may be made in the form orparts of the apparatus, and in the heating system disclosed withoutdeparting from the principles of the invention. For example, instead ofthe difierential spacing of the holes being spread "ormly over thestrainer, satisfactory liquor circulation may be obtained by providingthe strainer with a plurality of sep arate groups of perforations, theperforations in each group being on the same pitch with differentpitches in different groups and with the groups so arranged thatsubstantially the same result heretofore described is obtained. Also ifdesired, there may be two or more suction connections to the strainer,with the differential spacing of the perforations so applied as to givea uniform flow of liquor throughout the length of the strainer. Orfurther, the suction strainer might be of tubular construction having anoval or circular cross-section instead of triangular, as shown. 1

Accordingly, no intention is entertained to limit ply-invention exceptby the scope of the appended claims.

What I claim is:

1. Apparatus for pulp digestion comprising a digester, an annularsuction strainer of triangular crcss-section, means supporting saidsuction strainer on the inner wall of said digester, a circulating pump,a draw-oil conduit connecting the interior of said suction strainer tothe suction of said pump for withdrawing liquor from said digester,means for heating the liquor so withdrawn, means for returning a part ofsaid heated liquor to the upper portion of said digester, means forintroducing another part of said liquor into the lower portion of saiddigester with a whirling motion, the perforations in said suctionstrainer being arranged so that the eifective straining area isprogressively greater per unit of strainer area as the distance fromsaid drawoiT conduit increases.

2. Apparatus for pulp digestion comprising a digester and a tubularstrainer in the digester having an outlet arranged for connection to asource of suction, the apertures of the strainer being of substantiallythe same size and being arranged on a certain pitch immediately adjacentsaid outlet and on a pitch which progressively decreases from saidcertain pitch as the distance from said outlet increases to providesubstantially uniform flow through the strainer.

3. Apparatus for pulp digestion comprising a digester, an annularsuction strainer disposed in said digester, suction means connected tosaid strainer for withdrawing liquor from said digester, means forheating the liquor so withgreater per unit of strainer area as thedistance er area adjacent said outlet and a progressively greater numberof apertures per unit of strainer area as the distance from said outletincreases to provide substantially uniform flow through the strainer.

5. Apparatus for pulp digestion comprising a digester and a tubularstrainer adjacent an upright wall of the digester and conformingsubstantially to the contour of said wall, the strainer having anoutletvarranged for connection to a source of suction, the apertures ofthe strainer being of substantially the same size, there being a certainnumber of apertures per unit of strainer area in a portion of thestrainer adjacent said outlet and a greater number of apertures per unitof strainer area in other portions of said strainer to providesubstantially uniform flow through the strainer.

6. Apparatus for pulp digestion comprising a digester and a straineradjacent an upright wall of the digester and conforming substantially tothe contour of said wall and having a substantially triangulartransverse cross-sectional configuration, the strainer having an outletarranged for connection to a source of suction, the apertures of thestrainer being of substantially the same size, there being a certainnumber of apertures per unit of strainer area adjacent said outlet and aprogressively greater number of apertures per unit of strainer area asthe distance from said outlet increases to provide substantially uniformflow through the strainer. I

DION K. DEAN.

